The value of thermal barrier coatings on internal surfaces of engines is well recognized. For example, U.S. Pat. No. 4,495,907 issued Jan. 29, 1985 to Roy Kamo describes a thermally insulating coating, for combustion chamber components, composed of a plurality of metal oxides. After application of a bond coat, Kamo deposits a layer of thermally insulative material that is then impregnated with a chromium solution. Preferably the chromium solution penetrates substantially through the thermally insulative material and contacts the substrate. Upon heating, the chromium solution is converted to a refractory metal oxide that seals the surface of the thermally insulative material. This process requires a repetition of the impregnation and heating cycles, e.g., 5 or 6 times, to effect penetration of the impregnating solution. Not only is this process time consuming, and therefore costly, but impregnation of the thermally insulative material reduces the porosity of the insulative material and thereby compromises the thermal insulative properties of the coating.
A continuously graded metallic-ceramic coating for metallic substrates is disclosed in U.S. Pat. No. 4,588,607, issued May 13, 1986 to A. F. Matarese et al. The coating taught by this patent is applied to a metal substrate and includes a metallic bond coat, a continuously graded metallic-ceramic layer, and an abradable outer layer of ceramic material. During deposition of the coating, the metal substrate temperature is modulated to produce a desirably low residual stress pattern in the graded layer. This coating, however, does not provide an outer surface that is resistant to corrosion, erosion, or infiltration by the hot gases present in a combustion chamber during operation of an engine.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an effective thermal barrier coating for metal substrates that not only avoids high stresses at the interface of dissimilar materials, but also has an outer surface that is effectively sealed against infiltration of hot fuel gases. Furthermore, it is desirable to have such a thermal barrier coating in which the thermal insulating properties of the primary insulating material are not compromised by impregnation of a sealant.